We have found that Beta-endorphin induces in rats a profound state of immobilization, characterized by the absence of movement, loss of the righting response and extreme generalized muscular rigidity. A similar rigid immobility syndrome has also been observed by others. Opiates also induce a state of immobility which, especially at higher doses, is accompanied by muscular rigidity and loss of the righting response. These studies have been extended to include the characterization of the behavioral effects produced by the intraventricular administration of the human and porcine forms of Beta-endorphin as well as the metabolically resistant enkephalin analogs, D-Met2-Pro5-enkephalinamide and D-Ala2-Met5-enkephalinamide. The behavioral response profiles resulting from intraventricular and systemic injection of the opiates, morphine, methadone and etonitazene were also assessed. Futher, the neuroanatomical substrates mediating the various components of the immobility syndrome were examined by injection of opioid peptides and opiates into specific brain sites.